1. Field of the Invention
The invention relates generally to an apparatus and method for detecting abnormal air-fuel ratio variation among cylinders of a multi-cylinder internal combustion engine. More specifically, the invention relates to an apparatus and method for detecting relatively great air-fuel ratio variation among cylinders of a multi-cylinder internal combustion engine.
2. Description of the Related Art
In an internal combustion engine provided with an exhaust gas control system that uses a catalyst, it is usually necessary to control a mixture ratio between air and fuel, which constitute an air-fuel mixture that is burned in the internal combustion engine, that is, an air-fuel ratio, in order to remove toxic substances in the exhaust gas using the catalyst with high efficiency. To control the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine, and feedback control is executed in such a manner that the air-fuel ratio that is detected by the air-fuel ratio sensor agrees with a predetermined target air-fuel ratio.
In a multi-cylinder internal combustion engine, air-fuel ratio control is usually executed using the same control amount for all the cylinders. Therefore, even if the air-fuel ratio control is executed, the actual air-fuel ratio may vary among the cylinders. If the variation range is narrow, such small air-fuel ratio variation is absorbed by executing the air-fuel ratio feedback control, and toxic substances in the exhaust gas are removed by the catalyst. Therefore, such small air-fuel ratio variation does not exert an influence on the exhaust emission, and, therefore, does not cause a problem. However, if the air-fuel ratio greatly varies among the cylinders due to, for example, a malfunction of a fuel injection system of part of the cylinders, the exhaust emission deteriorates, which may cause a problem. Preferably, such great air-fuel ratio variation that may cause deterioration of the exhaust emission should be detected as an abnormality. Especially, in the case of an internal combustion engine for an automobile, detecting abnormal air-fuel ratio variation among cylinders using an on-board device is required in order to prevent a vehicle that emits deteriorated exhaust emission from running. Recently, there are moves for legislating provision of an on-board device that detects abnormal air-fuel ratio variation among the cylinders.
For example, a device described in Japanese Patent Application Publication No. 2004-176689 (JP-A-2004-176689) obtains a value indicating operating state variation among cylinders of an internal combustion engine based on, for example, an intake pipe pressure detected by an intake pipe pressure sensor.
It is newly revealed that there is a correlation between a range of air-fuel ratio variation among cylinders and a catalyst temperature.